1. Field of the Invention
The present invention relates to transformers and more particularly, to a low magnetic leakage high voltage transformer, which provides two output voltages to drive multiple loads.
2. Description of the Related Art
German scientist Karl Ferdinand Braun developed the first controllable CRT in 1897. However, it wasn't until the late 1940s that CRTs were used in the first television sets. Nowadays, various new display devices including TFT LCDs, PDPs, FPDs, etc. have been developed and have appeared on the market. It is predicated that TFT LCDs will soon completely take the place of CRTs on market. Currently, the backlight of a TFT LCD uses a number of CCFLs (Cold Cathode Fluorescent Lamps). In order to drive these CCFLs, each CCFL uses one respective circuit (electronic ballast). Because a CCFL discharges electricity, it produces negative resistance. When multiple CCFLs are connected in parallel to a transformer, the transformer can only turn on CCFL. Therefore, people usually put a rectifying capacitor between CCFLs and the respective transformers so that the CCFLs that are connected in parallel can be simultaneously turned on. However, because each CCFL uses a respective circuit (electronic ballast), the manufacturing cost of the TFT LCD is high, and the size of the TFT LCD cannot be reduced to meet the market trend toward light, thin, short and small.
In order to eliminate the aforesaid drawbacks, transformer manufacturers developed a one-to-multiple compact design of transformer. According to this design, a primary winding and multiple secondary windings are provided at two sides of a magnetic PATH, and a driving circuit is connected to the primary winding to provide electric energy to the primary winding, for enabling each secondary winding to output electric energy to a respective lamp via a respective capacitor. According to this design, one transformer is capable of driving multiple lamps. The use of such a transformer in a TFT LCD greatly saves the manufacturing cost and size of the TFT LCD. However, this design cannot eliminate magnetic interference between windings, and the secondary windings may output different amount of electric energy to the connected lamps, resulting in insufficient brightness of the lamps or damage to the lamps. In order to eliminate this problem, another design of high voltage transformer is developed. As illustrated in FIG. 11, the high voltage transformer comprises two electrically insulative brackets A arranged in parallel, primary windings A1 and secondary windings A2 respectively wound round the electrically insulative brackets A, and two U-shaped FERRITE cores B respectively inserted into the electrically insulative brackets A from two sides and set into abutment to form a magnetic PATH. This design of high voltage transformer can drive two lamp tubes at a time. This design of high voltage transformer uses multiple primary windings and secondary windings to drive multiple lamp tubes. However, because the windings use one common magnetic PATH, magnetic interference will occur between each two adjacent windings, resulting in unstable power output and uneven brightness of the connected lamp tubes. Further, the use of one common magnetic PATH by multiple primary windings and secondary windings will cause magnetic saturation, resulting in loss of magnetization and overheat of the FERRITE core set, and the power capacity of the high voltage transformer will drop.
Further, the arrangement of the multiple primary windings and secondary windings greatly increases the magnetic path length, resulting in a low leakage inductance and causing it unable to achieve the desired resonant frequency.
Therefore, it is desirable to provide a low magnetic leakage high voltage transformer that eliminates the aforesaid drawbacks.